MEASUREMENT OF THE V-T ENERGY-TRANSFER RATES OF HIGHLY EXCITED (2)A(1) NO2

Production of electronic ground state NO2 ((2)A(1)) from 248 nm photol ysis of HNO3 was detected by laser induced fluorescence (LIF). A growt h in the LIF signal was observed following the photolysis and has been interpreted as the relaxation of NO2 through the higher vibrational l evels of the X((2)A(1)) state; an energy region where the probe laser photodissociates the NO2 instead of inducing fluorescence. The rate co efficients for NO2 relaxation through these high vibrational levels we re determined by fits of time resolved LIF signal to a stepladder kine tic model. The results of the kinetic analysis suggest that the observ ed relaxation begins at the B-2(2) threshold near 9500 cm(-1) and exte nds downward through approximately 5 vibrational levels of the ground electronic surface. The derived quenching rate coefficients (in units of 10(-12) cm(3) molecule(-1) s(-1)) are 0.51+/-0.05, 1.0+/-0.1, 1.4+/ -0.2, 2.6+/-0.6, and 8.7+/-1.1 for Ar, He, N-2, O-2, and CO2 collision partners, respectively. The discrepancies between these coefficients and previous literature values are rationalized in terms of a dependen ce of the vibrational relaxation rate on total internal energy.